Abstract

Device-to-device (D2D) communications are recognized
as a key enabler of future cellular networks which will
help to drive improvements in spectral efficiency and assist
with the offload of network traffic. Among the transmission
modes of D2D communications are single-hop and relay assisted
multi-hop transmission. Relay-assisted D2D communications will
be essential when there is an extended distance between the
source and destination or when the transmit power of D2D user
equipments (UEs) is constrained below a certain level. Although
a number of works on relay-assisted D2D communications have
been presented in the literature, most of those assume that relay
nodes cooperate unequivocally. In reality, this cannot be assumed
since there is little incentive to cooperate without a guarantee of
future reciprocal behavior. Cooperation is a social behavior that
depends on various factors, such as peer comparison, incentives,
the cost to the donor and the benefit to the recipient. To
incorporate the social behavior of D2D relay nodes, we consider
the decision to relay using the donation game based on social
comparison and characterize the probability of cooperation in
an evolutionary context. We then apply this within a stochastic
geometric framework to evaluate the outage probability and
transmission capacity of relay assisted D2D communications.
Through numerical evaluations, we investigate the performance
gap between the ideal case of 100% cooperation and practical
scenarios with a lower cooperation probability. It shows that
practical scenarios achieve lower transmission capacity and
higher outage probability than idealistic network views which
assume full cooperation. After a sufficient number of generations,
however, the cooperation probability follows the natural rules of
evolution and the transmission performance of practical scenarios
approach that of the full cooperation case, indicating that all D2D
relay nodes adopt the same dominant cooperative strategy based
on social comparison, without the need for enforcement by an
external authority.